Pulse generator



April 24, 1951 A. H. BARTELS 2,549,764

PULSE GENERATOR Filed July 9, 1945 2' 2 Ill 0 5 3 INVENTOR *-||1||| ARNOLD H. BARTELS ATTORNEY Patented Apr. 24, 1951 PULSE GENERATOR Arnold H. Bartels, Fort mesne assignments, Corporation, a corpora Wayne, Ind., assignor, by

Farnsworth Research tion of Indiana Application July 9, 1945, Serial No. 603,976

1 Claim.

This invention relates generally to pulse generators, and particularly relates to a foreigndriven multivibrator circuit arranged to develop output pulses of short duration.

A multivibrator is an oscillator comprising two thermionic tubes having their plates cross-cc nected by impedances to their grids. In a conventional multivibrator condensers are utilized for cross-connecting the plates to the grids of the two tubes. A multivibrator of this type is usually self-oscillating, although it can be arranged not to be self-oscillating in which case driving impulses are required from an external source for determining'the frequency of the output signal. A circuit similar to a multivibrator has also been suggested where the plat=s are cross-connected to the grids by means of series resistors. When one of the tubes is conduct- .ing, the other tube is non-conducting. This condition is maintained until a driving impulse causes the space current in the conducting tube to cease, whereupon the previously non-conducting tube will conduct space current. Accordingly, two driving impulses are required for each operating cycle of the circuit.

Multivibrators have been widely used in the television and related fields for deriving square waves or pulses which may be employed, for example, for timing the scanning beams of television picture signal generating and reproducing tubes. In particular, multivibrators have been utilized for developing blanking and synchroniz ing pulses required for the transmission and reproduction of television pictures. For these purposes it is very desirable to utilize a pulse generator which develops pulses of short and controllable duration because the horizontal blanking pulses and the horizontal synchronizing pu ses occupy only approximately 15% and 8%, respectively, of the time period of an entire deflection cycle. Pulses of this type can be obtained from conventional multivibrators with difficulty only because both tubes of the multivibrator conduct space current at alternate times and each tube conducts space current for substantially the same period of time. When an output pulse of short duration is required one of the tubes of the multivibrator must be conductive for a short period of the entire cycle only which can not be conveniently effected with conventional circuits.

It is an object of the present invention, therefore, to provide a pulse generator for developing output pulses of short duration.

v A further object of the invention is to pro- 2 vide a novel foreign-driven multivihrator circuit for developing an output signal consisting of pulses of short controllable duration.

In accordance with the present invention, there is provided a pulse generator comprising a first electric discharge device arranged to be normally non-conducting and a second electric discharge device arranged to be normally conducting, each discharge device having a cathode, a grid and an anode. A resistor is provided for coupling the anode of the first discharge device to the grid of the second discharge device and a condenser is provided for coupling the anode of the second discharge device to the grid of the first discharge device. Means are provided for periodically rendering the first discharge device con-ducting and for simultaneously reducing the space current in the second discharge device. Finally, means are provided for deriving output pulses of short duration from the pulse generator.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. 7

In the accompanying drawing, the single figure shows a pulse generator comprising a multivibrator circuit embodying the present invention.

Referring now more particularly to the single figure of the drawing, there is illustrated a multivibrator comprising two thermionic tubes 5 and 2. Although, thermionic tubes i and 2 have been illustrated with separate envelopes they may also be enclosed in a common envelope in the manner of a duplex triode. Tube l includes cathode 3, control grid ii and anode while tube 2 is provided with cathode 6, control grid 1 and anode 8. Anode 5 of tube l is conductively connected to grid 7 of tube 2 through series resistor H! which may be variable as shown. Anode 8 of tube 2 is coupled to grid i of tube I through coupling condenser ll. Variable grid leak resistor 12 is provided for connecting grid 4 to ground.

Anodes 5 and 3 are supplied with positive potential from a suitable source indicated at 13+ through lead !3. Lead i3 is connected to anodes 5 and 8 through anode resistors i l and I5, respectively. Cathode 3 is connected to lead 53 and hence to the anode supply voltage through resistor 86. Resistor E'i connects cathode 3 to ground. Condenser l3 bypasses resistor i? to ground and provides a low impedance path for alternating current. The current flowing from the anode supply through resistors IE and ii to ground will cause a voltage drop across resistor I6 which will normally keep cathode 3,

positively biased with respect to ground. Accordingly, it will be seen that grid i is normally maintained at a negative potential with respect to cathode 3. Cathode "3 is connected to ground through cathode resistor 23. Condenser Z l, which may be variable as shown, may be provided for coupling grid 3 to anode Although condenser 2| may be used for controlling the duration of the output pulses, it is not essential for the operation of the multivibrator and may be omitted.

The input signal is developed by synchronizing signal source 25 coupled to control grid 4 through coupling condenser 26. The input signal. impressed upon grid 6 includes pulses of positive polarity with respect to ground as indicated at ll. The output signal may be obtained across cathode resistor 25 from lead 28. The output signal indicated at 36 consists of short pulses of negative polarity with respect to the normal potential of cathode 6, that is, when tube 2 is conducting.

At the present time no definite theory of operation of the multivibrator of the invention can be offered. It will be realized that it is very difiicult to measure experimentally the voltage and current waves which may be present, for I instance, on the electrodes of tubes I and 2. This is due to the fact that instrument such as a cathode ray oscilloscope which may be used for measuring the electric waves will introduce changes of the circuit constants of the multivibrator and thus change the operation thereof. The following tentative theory of operation is presented here with the understanding that it may not be correct in all details.

Let us assume that tube 2 has just been rendered conducting, for example, by connecting line [3 to the anode supply voltage B+. Consequently, control grid 7 will assume a positive potential equivalent to the anode supply voltage impressed through resistors M and it. Thus, grid 1' is positive with respect to cathode 5 and space current begins to flow through anode resistor l5 and tube 2. This causes a voltage drop across anode resistor l5 which will depress the potential of anode 8. grid 7 assumes momentarily a higher potential than anode 8 and, accordingly, grid 1 will draw grid current which flows from line l3 through resistors i and it through tube 2 to ground.

The grid current flowing through resistors Hi I and ill will depress the potential of control grid '1' until the anode current through tube 2 as well as the grid current have adjusted themselves to an equilibrium value so that the potential of grid 7 is below that of the anode supply voltage.

The depressed potential of anode 8 is impressed upon control grid 4 through coupling condenser H, thus biasing tube l beyond cutoif. However, the charge of coupling condenser H is gradually dissipated to ground through grid leak resistor I2. Therefore, the blocking potential impressed upon control grid c is removed after a period of time determined by the time constant of coupling condenser l l and grid leak resistor 52, However, tube I should be normally non-conducting and, therefore, a suitable biasing potential is supplied to tube l which will normally bias it beyond cutoff. This is efiected through the potential drop of the current flow- Thus, it will be seen that ing from the anode supply B+ through resistors i5 and I? to ground. Thus, cathode 3 is maintained at a potential that is positive with respect to ground and consequently control grid 4 is maintained negative with respect to cathode 3 because the potential of control grid 4 can not drop below ground potential.

Thus, it will be seen that tube 2 is normally conducting, while tube I is normally non-conducting. The multivibrator of the invention, therefore, is not self-oscillating and must be driven by a foreign source. For the purpose of developing one output pulse it is necessary to initiate the conduction of space current in tube l by some suitable means. This may be eifected, for instance, by synchronizing signal source 25 which develops positive driving pulses 2'. im pressed through coupling condenser 26 upon control grid 3. Driving pulses 21 should be of such a magnitude to overcome the negative bias between control grid 4 and cathode 3 sufficiently to render tube I conducting. Thus, upon the arrival of a positive driving pulse 2'! space current begins to flow in tube 1.

The space current of tube inflows through anode resistor i4 causing a voltage drop thereacross. The depressed potential of anode 5 is now impressed through resistor 18 upon control grid 1. The depressed potential of anode 5 is of such a value to drive control grid 1 sufficiently negative to either terminate space current through tube 2 entirely or to reduce it to a low value. Accordingly, either no space current or much less space current will flow through cathode resistor 28. As long as tube 2 is conducting space current a voltage drop is developed across cathode resistor 28 which will maintain cathode E at a positive potential with respect to ground. When the space current flowing through cathode resistor 20 is either terminated or considerably reduced, the potential of cathode 6 will approach ground potential. Accordingly, control grid 7 now assumes a less negative or even a positive potential with respect to cathode B so that space current is again initiated in tube 2.

The space current flowing through anode rcsistor it again causes a potential drop therecross which is impressed through coupling condenser upon control grid 4, thus biasing it beyond cutoff. In the meantime we may assume that the positive charge of condenser 26 due to the driving pulses 27 has been partly or entirely dissipated through grid leak resistor l2. The termination of the space current through tube 2 causes the potential of anode 5 to rise. This increased potential of anode 5 is impressed through resistor Ill upon control grid 7 which, in turn, increases the current flow through tube 2 still further. At this instant tube 2 conducts space current while tube 1 is non-conducting, and the cycle of operation repeats again upon the arrival of another positive driving pulse impressed upon control grid 4. Thus it will be seen that the multivibrator of the invention develops but one output pulse at a time in response to the arrival of a driving pulse from synchronizing signal source 25.

The output pulses may be derived across cathoie resistor 28 through output lead 28. It has been found experimentally that the duration of the output pulses is very short. This may easily be understood from the previous explanation of the operation because the space current through tube 2 is reduced or terminated only for a short Variable resistor in do period of the operating cycle. It has also been found experimentally that the duration of the output pulses may be controlled by adjusting grid leak resistor l2 and coupling resistor l0. It will be evident that grid leak resistor [2 in combination with coupling condenser I l controls the time constant of tube 1. The value of coupling resistor I determines the potential of control grid 1. Furthermore, it has been found that the duration of the output pulses may be controlled by adjusting coupling condenser Zl arranged between control grid 1 and anode 8.

While it will be understood that the circuit specifications of the multivibrator of the invention may vary according to the design of any particular application, the following circuit specifications for a multivibrator are included, by way of example only, suitable for a pulse frequency of the order of 14 kilocycles and using a duplex triode of the 6SN7 type:

Positive potential of anode supply B+ volts Condenser II micro-microfarads Condenser l8 microfarads Condenser 23 micro-microfarads Variable condenser 2| do Resistor M ohms Resistor I do Resistor do Resistor l5 ;do Resistor ll d0 4,700 Variable resistor l2 do 250,000

The pulse width obtained with the multivibrator of the invention having the above circuit specifications was approximately 7% of the operating cycle.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claim to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

A pulse generator comprising a first electric discharge device and a second electric discharge device, each having a cathode, a grid and an anode, and adjustable resistor for coupling the anode of said first device to the grid of said second device to control the duration of output pulses, a condenser for coupling the anode of said second device to the grid of said first device to control the time constant of said first discharge device, resistors connected to said anodes, means for supplying through said anode resistors a potential to said anodes that is positive with respect to ground, an adjustable grid leak resistor for connecting the grid of said first device to ground, means for normally maintaining the grid of said first device negative with respect to the cathode of said first device, an adjustable condenser for coupling grid and anode of said second device to control the duration of output pulses, another resistor for connecting the cathode of said second device to ground, means for periodically rendering said first device conducting, and means for deriving output pulses of short duration across the cathode resistor of said second device, said adjustable resistors and said adglustable condenser controlling the duration of said output pulses.

ARNOLD H. BARTELS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,262,838 Deloraine et a1 Nov. 18, 1941 2,338,395 Bartelink Jan. 4, 1944 2,399,135 Miller et al Apr. 23, 1946 2,402,916 Schroeder June 25, 1946 2,426,454 Johnson Aug. 26, 1947 2,431,832 Schlesinger -Dec. 2, 1947 OTHER REFERENCES Ultra-High-Frequency Techniques, Brainerd et al., May 24, 1943, Van Nostrand 00., Publisher, pages 176-177.

Journal of the Institution of Electrical Engineers, Part III, Communication Engineering, June 1942, page 110, Time Bases, by Puckle. 

